This invention relates to a ski structure and more specifically, it is concerned with at least one torsional reinforcing layer found between the top surface and the opposing bottom or running surface of the ski to obtain torsional stiffness in the ski.
The continued popularity of downhill skiing has focused attention on the structure of skis to produce a ski that provides greater responsiveness to the improved skiing techniques being employed by skiers today and the increased speed being achieved as a result of these techniques. This continued popularity has caused the materials used in skis to be changed in response to efforts to develop higher performance skis at lower manufacturing costs. Skis have been made solely from wood, composite wood-plastic materials, as well as entirely from plastics. Skis made entirely from metal have also been manufactured, as well as incorporating metal into composite wood-plastic skis or into all plastic skis. In particular, the advent of high performance wood-fiberglass and fiberglass-plastic foam skis has intensified the skiing industry's efforts to solve the problem of providing a ski constructed of quality materials which provide increased ski return rates, increased natural frequency, increased torsional stiffness, and a bottom steel running edge with increased impact resistance.
Different approaches have been taken in attempts to solve these problems as higher performance skis have evolved in the ski industry. Initially, skis were made with just a wooden core. A core made of plastic material, such as plastic foam or urethane, placed within a honeycomb structure formed from aluminum, was employed for a limited time. Historically, skis have been manufactured by laminating, torsion box or reaction injection molding processes. However, because of the higher performance nature of today's skis, these composite skis are subjected to greater flexibility strains which the aforementioned constructions have either failed to withstand or have provided skis which produce a dead sensation to the user.
None of the aforementioned structures have provided skis which balance the considerations of high material costs, difficulty in contouring the skis during manufacture and other problems and inefficiencies that occur during the molding and assembly processes employed in the manufacture of snow skis today. Similarly, no design has been able to maintain a desired flexual pattern, while allowing for a designed torsional response. The optimum or desired design employs a thinner ski that utilizes less material to produce a lighter and livelier ski.
Prior skis using a torsional reinforcing laminate layer typically employed a .+-.45 degree oriented bias ply fiberglass laminate. This type of a torsionally reinforcing layer has the predominant mechanical properties affecting torsional stiffness only along the .+-.45 degree axes, and not through a complete 360 degrees. This desire to obtain a truly isotropic torsional reinforcing layer, that is a layer that exhibits the same torsional stiffness values as measured by the modulus of elasticity of the laminate, when measured along axes in all directions or through 360 degrees was also spurred by the desire to reduce the thickness of the layer employed and the concomitant manufacturing costs. A truly isotropic reinforcing layer permits very minute torsional stiffness increases or decreases by varying the layer's thickness by as little as 0.1 millimeter or by altering the elastic modulus properties. These increases or decreases can be termed designably variable.
Use of .+-.45 degree oriented bias ply fiberglass laminate also can create substantial manufacturing problems if the fiberglass is not accurately oriented along the .+-.45 degree axes of the skis. Inaccurate orientation produces geometric warpage of the skis.
The foregoing problems are solved in the design of the present invention by providing a torsional reinforcing layer intermediate the top surface and the bottom running surface that employs randomly oriented fibers to obtain a livelier ski with increased torsional stiffness by optimizing the longitudinal flex, the vibrational characteristics and the torsional stiffness of the ski through adjustment of the thickness of the reinforcing laminate layer and the torsional stiffness of the layer based on the type and amount of the random fibers added to the reinforcing layer.